During the past few years, we have been able to form crystalline complexes containing a variety of drugs and nucleic acid constituents. Thus, actinomycin forms a crystalline complex with deoxyguanosine and the information afforded by the X-ray analysis of this structure led to an understanding of the binding of actinomycin to DNA. We have now succeeded in forming a large number of drug-nucleic acid crystalline complexes, notably ethidium, 9-aminoacridine, proflavin and acridine orange cocrystallized with the dinucleoside monophosphates, iodocytidyl(3'5') guanosine (iodoCpG). This information has led to unifying structural concepts in understanding a wide-range of drug-DNA interactions. A key concept that has emerged from these studies is the "kink" in DNA, a conformational flexibility in the polymer important for its physiological function. We have shown that this kink can be produced by altering the normal C2' endo sugar puckering in B DNA to a mixed sugar-puckering pattern of the type: C3' endo (3'-5') C2' endo and partially unstacking base-pairs. The ability of DNA to kink reflects the small energies needed for this conformational alteration. Such a conformational change in DNA could occur spontaneously at thermal energies and be associated with the normal modes in the DNA polymer. The concept of the kink is a central one in understanding the organization of DNA in chromatin. If one introduces the kink every 10 base-pairs in DNA, one gets a left-handed superhelical structure that could be used in the arrangement of DNA within the nucleosome structure. Higher order solenoidal structure can be achieved by altering the kink parameters. Further implications are discussed here.